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Title: PIPING BASICS


1
PIPING BASICS
  • Piping may be defined as a mode of transportation
    for the liquids, gases, and fluidized solids from
    one place to another. A pipe is basically a
    tubular structure which is specified by its
    nominal bore diameter and its Schedule number
    which generally gives the standards of its wall
    thickness. The piping is laid and designed
    according to some standards laid by some of the
    standard organizations of the world. It forms the
    heart of industries such as power plants,
    petrochemical projects etc.
  • Hence it becomes very important for us to know
    the designing of such structures. The sector
    where piping is used as the backbone are -
  • Oil Gas Industry
  • Refineries
  • Petro Chemicals
  • Chemical Plants
  • Power Plants
  • Water Treatment Plants
  • (vii) Pharmaceuticals Food Industry
  • (viii) Paper plants
  • In other few pages we shall gain the
    complete knowledge on piping and its basics.

2
OFFSHORE FUNDAMENTALS
INTRODUCTION Offshore industry is basically
concerned with the exploration, drilling and
production of oil and gas from the sea bed in
shallow as well as deep water. Oil and gas are
derived almost entirely from decayed plants and
bacteria. Energy from the sun, which fuelled the
plant growth, has been recycled into useful
energy in the form of hydrocarbon compounds -
hydrogen and carbon atoms linked
together. Offshore and gas originates from two
sources. Gas from beneath the southern North Sea
and the Irish Sea formed from coals which were
derived from the lush, tropical rain forests that
grew in the Carboniferous Period, about 300
million years ago. Oil and most gas under the
central and northern North Sea and west of the
Shet land Islands formed from the remains of
planktonic algae and bacteria that flourished in
tropical seas of the Jurassic and Cretaceous
Periods, about 140 to 130 million years ago (a
significant amount of the Kimmeridge Clay Format
ion is Cretaceous in age) . They accumulated in
muds, which are now the prolific Kimmeridge Clay
source rock. Crude oil is a complex mixture of
hydrocarbons with small amounts of other chemical
compounds that contain sulphur, nitrogen and
oxygen. Traces of other elements, such as sulphur
and nitrogen, were also present in the decaying
organic material, giving rise to small quantities
of other compounds in crude oil. Hydrocarbon
molecules come in a variety of shapes and sizes,
(straight -chain, branched chain or cyclic) ,
this is one of the things that makes them so
valuable because it allows them to be used in so
many different ways.
3
Oil and gas form as the result of a precise
sequence of environmental condit ions The
presence of organic material Organic remains
being t rapped and preserved in sediment The
material is buried deeply and then slowly
"cooked" by increased temperature and
pressure. Offshore platforms are used for
exploration of Oil and Gas from under Seabed and
processing. The First Offshore platform was
installed in 1947 off the coast of Louisiana in
6M depth of water. Today there are over 7,000
Offshore platforms around the world in water
depths up to 1,850M Platform size depends on
facilities to be installed on top side eg. Oil
rig, living quarters, Helipad etc. Classification
of water depths lt 350 M- Shallow water lt 1500 M
- Deep water gt 1500 M- Ultra deep water US
Mineral Management Service (MMS) classifies water
depths greater than 1,300 ft as deepwater, and
greater than 5,000 ft as ultra-deepwater.
4
OFFSHORE PROCESS
EXPLORATION DISCOVERING THE UNDERGROUND STRUCTURE
Large-scale geological structures that might hold
oil or gas reservoirs are invariably located
beneath non-productive rocks, and in addition
this is often below the sea. Geophysical methods
can penetrate them to produce a picture of the
pat tern of the hidden rocks. Relatively
inexpensive gravity and geomagnetic surveys can
identify potentially oil-bearing sedimentary
basins, but costly seismic surveys are essential
to discover oil and gas bearing structures.
Sedimentary rocks are generally of low density
and poorly magnetic, and are often underlain by
strongly magnetic, dense basement rocks. By
measuring anomalies or variations from the
regional average, a three-dimensional picture can
be calculated. Modern gravity surveys show a
generalized picture of the sedimentary basins.
Recently, high resolution aero-magnetic surveys
flown by specially equipped aircraft at 70 - 100m
altitude show fault t races and near surface
volcanic rocks. Shooting seismic surveys More
detailed in format ion about the rock layers
within such an area can be obtained by deep echo
sounding, or seismic reflect ion surveys. In
offshore areas these surveys are undertaken by a
ship (F52) towing both a submerged air or water
gun array, to produce short bursts of sound
energy, and a set of streamers of several
kilometers length. Each streamer contains a dense
array of hydrophone groups that collect and pass
to recorders echoes of sound from reflecting
layers.
5
The depths of the reflecting layers are
calculated from the time taken for the sound to
reach the hydrophones via the reflector this is
known as the two-way t ravel time (F50a b) .
The pulse of sound from the guns radiates out as
a hemispherical wave front , a port ion of which
will be reflected back towards the hydrophones
from rock interfaces (F50a) . The path of the
minute port ion of the reflected wave- front
intercepted by a hydrophone group is called a ray
path. Hydrophone groups spaced along the streamer
pick out ray paths that can be related to
specific points on the reflector surface (F50c) .
Graphs of the intensity of the recorded sound
plot ted against the two-way time are displayed
as wiggle t races (F50b) . Seismic recording at
sea always uses the common depth point (CDP)
method (F50c d) . A sequence of regularly
spaced seismic shot s is made as the survey
vessel accurately navigates its
course. Processing Processing recordings
involves many stages of signal processing and
computer summing. Firstly, wiggle t races from a
single CDP are collected into groups. Displayed
side by side in sequence they form a CDP gather
(F51a b) . Reflections from any one reflector
form a hyperbolic curve on the gather because the
sound takes longer to t ravel to the more distant
hydrophones. This effect is called normal move
out (NMO) . Correct ion is needed to bring the
pulses to a horizontal alignment , as if they all
came from vertically below the sound source
(F51c) . The separate wiggle t races are added
together, or stacked (F51d) . Stacking causes t
rue reflect ion pulses to enhance one another,
and hopefully, random noise will cancel out .
This process is repeated for all the CDPs on the
survey line. The stacked and corrected wiggle t
races are displayed side by side to give a
seismic sect ion (F51e)
6
SEISMIC SURVEYS
INTERPRETATION
7
Interpretation Seismic sect ions provide
2-dimensional views of underground structure. By
using special shooting techniques such as spaced
air gun arrays or towing the streamer slantwise,
or by shooting very closely spaced lines, it is
possible to produce 3-dimensional (3D) seismic
images (F59) . These images comprise vertical
sect ions and horizontal sect ions (
time-slices) . DRILLING DEALING WITH THE
UNDERGROUND STRUCTURE There are two basic types
of drilling rigs - fixed plat form rigs and
mobile rigs. Fixed platform rigs are installed on
large offshore plat forms and remain in place for
many years. Most of the large fields in the North
Sea such as Forties and Brent were developed
using fixed plat form rigs. Drilling fluid (also
called "mud") , which is mainly water-based, is
pumped continuously down the drill string while
drilling. It lubricates the drilling tools,
washes up rock cuttings and most importantly,
balances the pressure of fluids in the rock
format ions below to prevent blowouts. In
offshore drilling, the first step is to put down
a wide-diameter conductor pipe into the seabed to
guide the drilling and contain the drilling
fluid. It is drilled into the seabed from
semi-submersible rigs, but on product ion plat
forms a pile-driver may be used. As drilling
continues, completed sections of the well are
cased with steel pipe cemented into place. A
blowout preventer is attached to the top of the
casing. This is a stack of hydraulic rams which
can close off the well instantly if back pressure
(a kick) develops from invading oil, gas or
water. Drilling grinds up the rock into tea-
leaf-sized cuttings which are brought to the
surface by the drilling mud. The drilling mud is
passed over a shale shaker which sieves out the
cuttings .
8
DRILLING
9
In exploration drilling, the cuttings are taken
for examination by a geologist known as a mud
logger who is constantly on the lookout for oil
and gas. PRODUCTION THE OFFSHORE
CHALLENGE Production facilities had to be
designed to withstand wind gusts of 180 km/ hour
and waves 30 met res high. Other problems
included the ever-present salt -water corrosion
and fouling by marine organisms. Dealing with the
many underwater construction and maintenance
tasks falls to divers and remotely operated
vehicles. Giant floating cranes (F83) designed to
lift ever greater loads were commissioned and
many other specialized craft had to be developed
to establish and service the offshore industry.
Huge helicopter fleet s were needed to ferry
workers to and from the plat forms and
rigs. Product ion Plat forms Most oil and gas
product ion plat forms in offshore Britain rest
on steel supports known as jackets, a term
derived from the Gulf of Mexico. A small number
of plat forms are fabricated from concrete. The
steel jacket , fabricated from welded pipe, is
pinned to the sea floor with steel piles. Above
it are prefabricated units or modules providing
accommodation and housing various facilities
including gas turbine generating sets. Towering
above the modules are the drilling rig derrick (
two on some plat forms) , the flare stack in some
designs (also frequently cantilevered outwards)
and service cranes. Horizontal surfaces are taken
up by store areas, drilling pipe deck and the
vital helicopter pad. Concrete gravity plat forms
are so- called because their great weight holds
them firmly on the seabed. They were first
developed to provide storage capacity in
oilfields where tankers were used to transport
oil, and to eliminate the need for piling in hard
sea beds.
10
The Brent D plat form (F87) , which weighs more
than 200 000 tonnes, was designed to store over a
million barrels of oil. But steel plat forms, in
which there have been design advances, are now
favored over concrete ones. Several plat forms
may have to be installed to exploit the larger
fields, but where the capacity of an existing
plat form permits, subsea collecting systems
linked to it by pipelines have been developed
using the most modern technology. They will be
increasingly used as smaller fields are
developed. For very deep waters, one solution was
the Hut ton Tension Leg Plat form the buoyant
plat form, resembling a huge drilling rig, is
tethered to the sea-bed by jointed legs kept in
tension by computer- cont rolled ballast
adjustments. Alternatively, a subsea collect ion
system may be linked via a product ion riser to a
Floating, Production, Storage and Offloading
(FSPO) vessel (F88) either a purpose built ship
or a converted tanker or semisubmersible rig. The
oil is offloaded by a shut t le tanker. Product
ion Wells To develop offshore fields as
economically as possible, numerous directional
wells radiate out from a single plat form to
drain a large area of reservoir (F94) . For
directional drilling special weighted drill
collars are used with a bent sub to deflect the
drill bit at a certain angle in the required
direct ion (F93) . Wells which deviate at more
than 65 degrees from the vertical and reach out
horizontally more than twice their vertical depth
are known as extended reach wells. More than one
horizontal sect ion can be drilled in one well as
a multilateral well (F96) . This technique is
used to reduce drilling costs and to maximize the
number of wells that can be drilled from small
plat forms.
11
Semi-Submersible
T.L.P.
Platform
Land Rig
Jack-up
Drill Ship
PRODUCTION
12
GETTING OIL AND GAS ASHORE Most offshore oil and
all offshore gas are brought to shore by
pipelines which operate in all weathers. Pipeline
routes are planned to be as short as possible.
Slopes that could put stress on unsupported pipe
are avoided and seabed sediments are mapped to
identify unstable areas and to see if it will be
possible to bury the pipe. Pipeline construction
begins onshore, as lengths of pipe are
waterproofed with bitumen and coated with steel-
reinforced concrete. This coating weighs down the
submarine pipeline even when it is filled with
gas. The prepared pipe- lengths are welded
together offshore on a lay barge (F101) . As the
barge winches forward on its anchor lines, the
pipeline drops gently to the seabed, guided by a
stinger. The inside of pipelines need to be
cleaned regularly to remove wax deposits and
water to do this a collecting device known as a
pig is forced through the pipe. Where tankers
transport oil from small or isolated fields,
various oil storage systems may be used. These
may range from cylindrical cells contained in
some of the massive concrete structures, to
seabed storage units such as that employed at the
Kittiwake field, or integral storage such as that
contained in the various Floating, Product ion,
Storage and Offloading vessels. In essence these
FPSOs are floating storage tankers, as well as
product ion and processing installations. FPSOs
provide an important option for developing fields
which may be remote from existing infrastructure
or where the field recoverable reserves are
uncertain, for example because of difficult
geological conditions.
13
OIL PLATFORMS
  • An oil platform or oil rig is a large structure
    used to house workers and machinery needed to
    drill and/or extract oil and natural gas through
    wells in the ocean bed. Depending on the
    circumstances, the platform may be attached to
    the ocean floor, consist of an artificial island,
    or be floating. Generally, oil platforms are
    located on the continental shelf, though as
    technology improves and crude oil prices
    increase, drilling and production in deeper
    waters becomes both feasible and profitable. A
    typical platform may have around thirty wellheads
    located on the platform and directional drilling
    allows reservoirs to be accessed at both
    different depths and at remote positions up to 5
    miles (8 kilometres) from the platform. Many
    platforms also have remote wellheads attached by
    umbilical connections, these may be single wells
    or a manifold centre for multiple wells.
  • Offshore platforms can broadly be categorized
  • into two parts -
  • Structures that extend to the sea bed
  • Jacketed or Fixed Steel platform
  • Concrete Gravity Structures
  • Compliant Tower
  • Structures that float near the water surface-
    Recent Development
  • Tension Leg Platforms
  • SPAR
  • Ship shaped Vessels (FPSO)

14
STRUCTURES THAT EXTEND TO THE SEA BED
FIXED STEEL PLATFORMS A Fixed Platform is
a type of offshore platform used for the
production of oil or gas. These platforms are
built on concrete and/or steel legs anchored
directly onto the seabed, supporting a deck with
space for drilling rigs, production facilities
and crew quarters. Such platforms are, by virtue
of their immobility, designed for very long term
use (for instance the Hibernia platform). Various
types of structure are used, steel jacket,
concrete caisson, floating steel and even
floating concrete. Steel jackets are vertical
sections made of tubular steel members, and are
usually piled into the seabed. Concrete caisson
structures, pioneered by the Condeep concept,
often have in-built oil storage in tanks below
the sea surface and these tanks were often used
as a flotation capability, allowing them to be
built close to shore (Norwegian Fjords and
Scottish Firths are popular because they are
sheltered and deep enough) and then floated to
their final position where they are sunk to the
seabed. Fixed platforms are economically feasible
for installation in water depths up to about
1,700 feet (520 m). Space framed structure with
tubular members supported on piled foundations.
Used for moderate water depths up to 400 M.
Jackets provides protective layer around the
pipes. Typical offshore structure will have a
deck structure containing a Main Deck, a Cellar
Deck, and a Helideck. The deck structure is
supported by deck legs connected to the top of
the piles. The piles extend from above the Mean
Low Water through the seabed and into the soil.
Underwater, the piles are contained inside the
legs of a jacket structure which serves as
bracing for the piles against lateral loads.
15
FIXED STEEL PLATFORMS
16
The jacket also serves as a template for the
initial driving of the piles. (The piles are
driven through the inside of the legs of the
jacket structure).Natural period (usually 2.5
second) is kept below wave period (14 to 20
seconds) to avoid amplification of wave loads.
95 of offshore platforms around the world are
Jacket supported. CONCRETE GRAVITY BASE
STRUCTURES Whilst the vast majority of fixed
offshore platforms employ a tubular jacket to
support the topside facilities, a number of
installations have been constructed using a base
manufactured from reinforced concrete. They are
Fixed-bottom structures made from concrete . They
are heavy and remain in place on the seabed
without the need for piles. They are widely used
for moderate water depths up to 300 M. Part
construction is made in a dry dock adjacent to
the sea. The structure is built from bottom up,
like onshore structure. At a certain point , dock
is flooded and the partially built structure
floats. It is towed to deeper sheltered water
where remaining construction is completed. After
towing to field, base is filled wi1th water to
sink it on the seabed. Its main advantage is its
less maintenance. The first concrete structure to
be installed in the North Sea was constructed by
the Norwegians in 1973 and used to develop the
Ekofisk field. Since then those people have
installed a steady string of concrete structures
and it came as no surprise when their government
elected to develop the other fields with the same
concrete structure which stands in 350ft. of
water and is currently the largest offshore
structure in Europe and the largest concrete
platform in the world.  
17
CONCRETE GRAVITY BASE STRUCTURE
COMPLIANT TOWER
18
COMPLIANT TOWER A compliant tower (CT) is a
fixed rig structure normally used for the
offshore production of oil or gas. The rig
consist of narrow, flexible (compliant) towers
and a piled foundation supporting a conventional
deck for drilling and production operations.
Compliant towers are designed to sustain
significant lateral deflections and forces, and
are typically used in water depths ranging from
1,500 and 3,000 feet (450 and 900 m). With the
use of flex elements such as flex legs or axial
tubes, resonance is reduced and wave forces are
de-amplified. This type of rig structure can be
configured to adapt to existing fabrication and
installation equipment. Compared with floating
systems, such as Tension leg platforms and SPARs,
the production risers are conventional and are
subjected to less structural demands and flexing.
This flexibility allows it to operate in much
deeper water, as it can 'absorb' much of the
pressure exerted on it by the wind and sea.
Despite its flexibility, the compliant tower
system is strong enough to withstand hurricane
conditions. The first tower emerged in the early
1980s with the installation of Exxon's Lena oil
platform. Narrow, flexible framed structures
supported by piled foundations. It has no oil
storage capacity. Production is through tensioned
rigid risers and export by flexible or catenary
steel pipe. It undergos large lateral deflections
(up to 10 ft) under wave loading. Used for
moderate water depths up to 600 M. Natural period
(usually 30 second) is kept above wave period (14
to 20 seconds) to avoid amplification of wave
loads.
19
STRUCTURES THAT FLOAT NEAR THE WATER
SURFACE TENSION LEG PLATFORMS A Tension-leg
platform or Extended Tension Leg Platform (ETLP)
is a vertically moored floating structure
normally used for the offshore production of oil
or gas and is particularly suited for water
depths greater than 300 meters (about 1000 ft).
Also proposed for wind turbines. The platform is
permanently moored by means of tethers or tendons
grouped at each of the structure's corners. A
group of tethers is called a tension leg. A
feature of the design of the tethers is that they
have relatively high axial stiffness(low
elasticity), such that virtually all vertical
motion of the platform is eliminated. This allows
the platform to have the production wellheads on
deck (connected directly to the subsea wells by
rigid risers), instead of on the seafloor . This
makes for a cheaper well completion and gives
better control over the production from the oil
or gas reservoir. The first Tension Leg Platform
was built for Conoco's Hutton field in the North
Sea in the early 1980s. The hull was built in the
dry-dock at Highland Fabricator's Nigg yard in
the north of Scotland, with the deck section
built nearby at McDermott's yard at Ardersier.
The two parts were mated in the Moray Firth in
1984. Tension Leg Platforms (TLPs) are floating
facilities that are tied down to the seabed by
vertical steel tubes called tethers. This
characteristic makes the structure very rigid in
the vertical direction and very flexible in the
horizontal plane. The vertical rigidity helps to
tie in wells for production, while, the
horizontal compliance makes the platform
insensitive to the primary effect of waves. It
has large columns and Pontoons and a fairly deep
draught.
20
TLP has excess buoyancy which keeps tethers in
tension. Topside facilities , no. of risers etc.
have to fixed at pre-design stage. It is used for
deep water up to 1200 M. It has no integral
storage. It is sensitive to topside load/draught
variations as tether tensions are affected.
SPAR A SPAR, named for logs used as buoys in
shipping and moored in place vertically, is a
type of floating oil platform typically used in
very deep waters. Spar production platforms have
been developed as an alternative to conventional
platforms. A Spar platform consists of a
large-diameter, single vertical cylinder
supporting a deck. It contains a deep-draft
floating caisson, which is a hollow cylindrical
structure similar to a very large buoy. Its four
major systems are hull, moorings, topsides and
risers. About 90 of the structure is underwater.
The spar design is now being used for drilling,
production, or both. The distinguishing feature
of a spar is its deep-draft hull, which produces
very favorable motion characteristics compared to
other floating concepts. Water depth capability
has been stated by industry as ranging up to
10,000 ft. The first Spar platform in the was
installed in September of 1996. It follows the
concept of a large diameter single vertical
cylinder supporting deck. These are a very new
and emerging concept the first spar platform,
Neptune, was installed off the USA coast in
1997.Spar platforms have taut catenary moorings
and deep draught, hence heave natural period is
about 30 seconds.
21
TENSION LEG PLATFORM
SPAR
22
FPSO Floating Production Storage and
Offloading A Floating Production, Storage and
Offloading vessel (FPSO also called a "unit" and
a "system") is a type of floating tank system
used by the offshore oil and gas industry and
designed to take all of the oil or gas produced
from a nearby platform (s), process it, and store
it until the oil or gas can be offloaded onto
waiting tankers or sent through a
pipeline.    History Oil has been produced from
offshore locations since the 1950s. Originally,
all oil platforms sat on the seabed, but as
exploration moved to deeper waters and more
distant locations in the 1970s, floating
production systems came to be used. The first oil
FPSO was the Shell Castellon, built in Spain in
1977. The first ever conversion of a LNG carrier
(Golar LNG owned Moss type LNG carrier) into an
LNG floating storage and regasification unit was
carried out in 2007 by Keppel shipyard in
Singapore. The last few years concepts for LNG
FPSOs has also been launched. An LNG FPSO works
under the same principles as an Oil FPSO, but it
only produces natural gas, condensate and/or LPG,
which is stored and offloaded.
23
Working principles Oil produced from offshore
production platforms can be transported to the
mainland either by pipeline or by tanker. When a
tanker solution is chosen, it is necessary to
accumulate oil in some form of tank such that an
oil tanker is not continuously occupied while
sufficient oil is produced to fill the
tanker. Often the solution is a decommissioned
oil tanker which has been stripped down and
equipped with facilities to be connected to a
mooring buoy. Oil is accumulated in the FPSO
until there is sufficient amount to fill a
transport tanker, at which point the transport
tanker connects to the stern of the floating
storage unit and offloads the oil. An FPSO has
the capability to carry out some form of oil
separation process obviating the need for such
facilities to be located on an oil platform.
Partial separation may still be done on the oil
platform to increase the oil capacity of the
pipeline(s) to the FPSO. Advantages Floating
Production, Storage and Offloading vessels are
particularly effective in remote or deepwater
locations where seabed pipelines are not cost
effective. FPSOs eliminate the need to lay
expensive long-distance pipelines from the oil
well to an onshore terminal. They can also be
used economically in smaller oil fields which can
be exhausted in a few years and do not justify
the expense of installing a fixed oil platform.
Once the field is depleted, the FPSO can be moved
to a new location.
24
FPSO
25
Specific types A Floating Storage and
Offloading unit (FSO) is a floating storage
device, which is simplified FPSO without the
possibility for oil or gas processing. Most FSOs
are old single hull supertankers that have been
converted. An example of this is the Knock Navis,
the world's largest ship, which has been
converted to an FSO to be used offshore Qatar. A
LNG floating storage and regasification unit
(FSRU) is a floating storage and regasification
system, which receives liquefied natural gas(LNG)
from offloading LNG carriers, and the onboard
regasification system provides natural gas
send-out through flexible risers and pipeline to
shore.
26
  • MODUs Mobile Offshore Drilling Units
  • The basic work of the mobile units is to drill
    the well in the sea bed and prepare the line for
    production. Offshore drilling is divided into two
    parts i.e shallow water and deep water. In
    shallow water, jack-up rigs, standing with their
    feet on the seabed are used to drill the oil
    wells. In deeper water, floating drilling units
    are used. There are two basic types Drill ships
    and Semi-submersible drilling rigs. This is a
    very important process and is very hard in nature
    as the environmental conditions are always
    unfavorable for such a process to accomplish.
    There are basically three type of drilling units
    that are widely used over the world. They are -
  • Semi-submersible drill rigs
  • Self elevated Jack up rigs
  • Drill ships

27
SEMI SUBMERSIBLE DRILLING RIG
  • A semi-submersible drilling rig is one in which
  • Sea water is pumped into the hull of the vessel
    causing the vessel to submerge to the
    desired depth.
  • The submerged vessel maintains its position over
    the well location by means of anchor chains.
  • A semi-submersible is not bottom-founded and can
    work in much greater water depths than a jack-up.
  • The maximum water depth is a function of the
    length of the rig's riser, a bundle of utility
    tubes through which drilling fluids and other
    material is conducted, enclosed in an outer tube,
    suspended to the seafloor.

DRILL SHIPS
Drill ships, a maritime vessel that has been
fitted with drilling apparatus. It is most often
used for exploratory drilling of new oil or gas
wells in deep water but can also be used for
scientific drilling. It is often built on a
modified tanker hull and outfitted with a dynamic
positioning system to maintain its position over
the well.
28
SEMI SUBMERSIBLE DRILL RIGS
DRILL SHIPS
29
SELF ELEVATED JACK UP RIGS
30
ELEVATING JACK UP RIGS
INTRODUCTION A Jack Up is an offshore structure
composed of a hull, legs and a lifting system
that allows it to be towed to a site, lower its
legs into the seabed and elevate its hull to
provide a stable work deck capable of
withstanding the environmental loads. A typical
modern drilling Jack Up is capable of working in
harsh environments (Wave Heights up to 80 ft,
Wind Speeds in excess of 100 knots) in water
depths up to 500 feet. Because Jack Ups are
supported by the seabed, they are preloaded when
they first arrive at a site to simulate the
maximum expected leg loads and ensure that, after
they are Jacked to full air gap and experience
operating and environmental loads, the supporting
soil will provide a reliable foundation. Jack Up
Units have been a part of the Offshore Oil
Industry exploration fleet since the 1950s. They
have been used for exploration drilling, tender
assisted drilling, production, accommodation, and
work/maintenance platforms. As with every
innovative technology, Jack Up Units have been
used to their operational and design limitations.
These limitations include deck load carrying
limits when afloat, load carrying capabilities
when elevated, environmental limits, drilling
limits, and soil (foundation) limits.
31
The reasons for pushing these limits include the
desire to explore deeper waters, deeper
reservoirs in harsher environments, and in areas
where soils and foundations may be challenging
or even unstable. There are three main
components of a Jack Up Unit the Hull, the Legs
Footings, and the Equipment. Each of the
component are described below - HULL The Hull
of a Jack Up Unit is a watertight structure that
supports or houses the equipment, systems, and
personnel, thus enabling the Jack Up Unit to
perform its tasks. When the Jack Up Unit is
afloat, the hull provides buoyancy and supports
the weight of the legs and footings (spud cans),
equipment, and variable load. Different
parameters of the hull affect different modes of
operation of the Unit. In general, the larger the
length and breadth of the hull, the more variable
deck load and equipment the Unit will be able to
carry, especially in the Afloat mode (due to
increased deck space and increased
buoyancy). Also, larger hulls generally result in
roomier machinery spaces and more clear space on
the main deck to store pipe, 3rd Party Equipment,
and provide for clear work areas. The larger hull
may have larger preload capacity that may permit
increased flexibility in preloading operations.
Larger hulls generally have the negative effects
of attracting higher wind, wave and current
loads. Since Jack Ups with larger hulls weigh
more, they will require more elevating jacks of
larger capacity to elevate and hold the Unit.
32
The large weight also affects the natural period
of the Jack Up Unit in the elevated mode. The
draft of the hull, or the distance from the
afloat waterline to the baseline of the hull, has
a direct effect on the amount of variable deck
load that can be carried and the stability when
afloat. The draft of the hull has an opposing
relationship with the hulls freeboard, or the
distance from the afloat waterline to the main
deck of the hull. Every incremental increase in
the draft of a Jack Up decreases the freeboard by
the same increment. LEGS AND FOOTINGS The legs
and footings of a Jack Up are steel structures
that support the hull when the Unit is in the
Elevated mode and provide stability to resist
lateral loads. Footings are needed to increase
the soil bearing area thereby reducing required
soil strength. The legs and footings have certain
characteristics which affect how the Unit reacts
in the Elevated and Afloat Modes, while going on
location and in non-design events. The legs of a
Jack Up Unit may extend over 500 ft above the
surface of the water when the Unit is being towed
with the legs fully retracted. Depending on size
and length, the legs usually have the most
detrimental impact on the afloat stability of the
Unit. The heavy weight at a high center of
gravity and the large wind area of the legs
combine to dramatically affect the Units afloat
stability. For Units of the same hull
configuration and draft, the Unit with the larger
legs will have less afloat stability. When in the
Elevated Mode, the legs of a Jack Up Unit are
subjected to wind, wave, and current loadings. In
addition to the specifics of the environment, the
magnitude and proportion of these loads is a
function of the water depth, air gap (distance
from the water line to the hull baseline) and the
distance the footings penetrate into the seabed.
33
Generally, the larger the legs and footings, the
more load wind, wave, and current will exert on
them. Legs of different design and size exhibit
different levels of lateral stiffness (amount of
load needed to produce a unit deflection). Jack
Up stiffness decreases with increases in water
depth (or more precisely, with the distance from
the support footing to the hull/leg connection).
Furthermore, for deeper water depths, flexural
stiffness (chord area and spacing) overshadows
the effects of shear stiffness (brace). Leg
stiffness is directly related to Jack Up
stiffness in the elevated mode, thereby affecting
the amount of hull sway and the natural period of
the Unit (which may result in a magnification of
the oscillatory wave loads). EQUIPMENT The
equipment required to satisfy the mission of the
Jack Up Unit affects both the hull size and
lightship weight of the Unit. There are three
main groups of equipment on a Jack Up Unit, the
Marine Equipment, Mission Equipment, and
Elevating Equipment. Marine Equipment refers to
the equipment and systems aboard a Jack Up Unit
that are not related to the Mission Equipment.
Marine Equipment could be found on any sea-going
vessel, regardless of its form or function.
Marine Equipment may include items such as main
diesel engines, fuel oil piping, electrical power
distribution switchboards, lifeboats, radar,
communication equipment, galley equipment, etc.
Marine Equipment, while not directly involved
with the Mission of the Jack up Unit, is
necessary for the support of the personnel and
equipment necessary to carry out the Mission. All
Marine Equipment is classified as part of the
Jack Up Lightship Weight.
34
Mission Equipment refers to the equipment and
systems aboard a Jack Up Unit, which are
necessary for the Jack Up to complete its
Mission. Mission Equipment varies by the mission
and by the Jack Up. Two Jack Up Units which are
involved in Exploration Drilling may not have the
same Mission Equipment. Examples of Mission
Equipment may include derricks, mud pumps, mud
piping, drilling control systems, production
equipment, cranes, combustible gas detection and
alarms systems, etc. Mission Equipment is not
always classified as part of the Jack Up
Lightship Weight. Some items, such as cement
units, are typically classified as variable deck
load as they may not always be located aboard the
Jack Up. MODES OF OPERATION OF A JACK UP Jack
Up Units operate in three main modes transit
from one location to another, elevated on its
legs, and jacking up or down between afloat and
elevated modes. Each of these modes has specific
precautions and requirements to be followed to
ensure smooth operations. A brief discussion of
these modes of operations along with key issues
associated with each follows. TRANSIT FROM ONE
LOCATION TO ANOTHER The Transit Mode occurs when
a Jack Up Unit is to be transported from one
location to another. Transit can occur either
afloat on the Jack Up Units own hull (wet tow),
or with the Jack Up Unit as cargo on the deck of
another vessel (dry tow). .
35
Main preparations for each Transit Mode address
support of the legs, support of the hull,
watertight integrity of the unit, and stowage of
cargo and equipment to prevent shifting due to
motions. Though the Units legs must be raised to
ensure they clear the seabed during tow, it is
not required that the legs be fully retracted.
Allowing part of the legs to be lower than the
hull baseline not only reduces jacking time, but
it also reduces leg inertia loads due to tow
motions and increases stability due to decreased
wind overturning. Lowering the legs a small
distance may also improve the hydrodynamic flow
around the open leg wells and reduce tow
resistance. Whatever the position of the legs
during tow, their structure at the leg/hull
interface must be checked to ensure the legs can
withstand the gravity and inertial loads
associated with the tow. Field Tow corresponds to
the condition where a Jack Up Unit is afloat on
its own hull with its legs raised, and is moved a
relatively short distance to another location.
For a short move, the ability to predict the
condition of the weather and sea state is
relatively good. Therefore, steps to prepare the
Unit for Field Tow are not as stringent as for a
longer tow. Most Classification Societies define
a Field Tow as a Tow that does not take longer
than 12 hours, and must satisfy
certain requirements with regards to motion
criteria. This motion criterion, expressed as a
roll/pitch magnitude at a certain period, limits
the inertial loads on the legs and leg support
mechanism. For certain moves lasting more than 12
hours, a Unit may undertake an Extended Field
Tow. An Extended Field Tow is defined as a Tow
where the Unit is always within a 12-hour Tow of
a safe haven, should weather deteriorate. In this
condition, the Jack Up Unit is afloat on its own
hull with its legs raised, similar to a Field
Tow. The duration of an Extended Field Tow may be
many days. The motion criteria for an Extended
Field Tow is the same as for a Field Tow.
36
The main preparations for a Unit to undertake an
Extended Field Tow are the same as those for a
Field Tow with the additional criteria that the
weather is to be carefully monitored throughout
the duration of the tow. A Wet Ocean Tow is
defined as an afloat move lasting more than
12-hours which does not satisfy the requirements
of an Extended Field Tow. In this condition, the
Jack Up Unit is afloat on its own hull with its
legs raised as with a Field Tow, but, for many
Units, additional precautions must be made. This
is because the motion criteria for a Wet Ocean
Tow are more stringent than for a Field Tow. The
additional preparations may include installing
additional leg supports, shortening the leg by
cutting or lowering, and securing more equipment
and cargo in and on the hull. A Dry Ocean Tow is
defined as the transportation of a Jack Up Unit
on the deck of another vessel. In this condition,
the Jack Up Unit is not afloat, but is secured as
deck cargo. The motion criteria for the Unit is
dictated by the motions of the transportation
vessel with the Unit on board. Therefore, the
precautions to be taken with regard to support of
the legs must be investigated on a case-by-case
basis. Generally, though, the legs are to be
retracted as far as possible into the hull so the
Jack Up hull can be kept as low as practicable to
the deck of the transport vessel and to reduce
the amount of cribbing support. The other
critical precaution unique to Dry Ocean Tow is
the support of the Jack Up hull. The hull must be
supported by cribbing on strong points
(bulkheads) within the hull and in many cases,
portions of the hull overhang the side of the
transportation vessel. These overhanging sections
may be exposed to wave impact, putting additional
stress on the hull, and if the overhanging
sections include the legs, the resultant bending
moment applied to the hull (and amplified by
vessel motions) can be significant. Calculations
should be made to ensure that the hull will not
lift off the cribbing with the expected tow
motions.
37
DIGRAMETIC VIEW OF THE OPERATION OF JACK UP RIG
38
ARRIVING ON LOCATION Upon completion of the
Transit Mode, the Jack Up Unit is said to be in
the Arriving On Location Mode. In this Mode,
the Unit is secured from Transit Mode and begins
preparations to Jack Up to the Elevated Mode.
Preparations include removing any wedges in the
leg guides, energizing the jacking system, and
removing any leg securing mechanisms
installed for the Transit thereby transferring
the weight of the legs to the pinions. SOFT
PINNING THE LEGS If an independent leg Jack Up
Unit is going to be operated next to a Fixed
Structure, or in a difficult area with bottom
restrictions, the Jack Up Unit will often be
temporarily positioned just away from its final
working location. This is called Soft Pinning
the legs or Standing Off location. This
procedure involves lowering one or more legs
until the bottom of the spud can(s) just touches
the soil. The purpose of this is to provide a
Stop point in the Arriving On Location process.
Here, all preparations can be checked and made
for the final approach to the working location.
This includes coordinating with the assisting
tugs, running anchor lines to be able to winch
in to final location, powering up of positioning
thrusters on the Unit (if fitted), checking the
weather forecast for the period of preloading and
jacking up, etc.
39
FINAL GOING ON LOCATION Whether a Unit stops
at a Soft Pin location, or proceeds directly to
the final jacking up location, they will have
some means of positioning the Unit so that
ballasting or preloading operations prior to
jacking up can commence. For an independent leg
Jack Up Unit, holding position is accomplished by
going on location with all three legs lowered so
the bottom of the spud can is just above the
seabed. When the Unit is positioned at its final
location, the legs are lowered until they can
hold the rig on location without the assistance
of tugs. Mat type Jack Up Units are either held
on location by tugs, or they drop spud piles into
the soil. These spud piles, usually cylindrical
piles with concrete fill, hold the Unit on
location until the mat can be ballasted and
lowered. JACKING A mat Unit will jack the mat
to the seabed in accordance with the ballasting
procedure. Once the mat has been lowered to the
seabed, the hull will be jacked out of the water.
The Unit then proceeds to Preload Operations .
All Independent leg Units must perform Preload
Operations before they can jack to the design air
gap. Most independent leg Units do not have the
capacity to elevate the Unit while the preload
weight is on board. For these Units, the next
step is to jack the hull out of the water to a
small air gap that just clears the wave crest
height. This air gap should be no more than five
(5) feet. Once they reach this position, the Unit
may proceed with Preload Operations.
40
PRELOAD OPERATIONS All Jack Up Units must
load the soil that supports them to the full load
expected to be exerted on the soil during the
most severe condition, usually Storm Survival
Mode. This preloading reduces the likelihood of a
foundation shift or failure during a Storm. The
possibility does exist that a soil failure or leg
shift may occur during Preload Operations. To
alleviate the potentially catastrophic results of
such an occurrence, the hull is kept as close to
the waterline as possible, without incurring wave
impact. Should a soil failure or leg shift occur,
the leg that experiences the failure loses
load-carrying capability and rapidly moves
downward, bringing the hull into the water. Some
of the load previously carried by the leg
experiencing the failure is transferred to the
other legs potentially overloading them. The leg
experiencing the failure will continue to
penetrate until either the soil is able to
support the leg, or the hull enters the water to
a point where the hull buoyancy will provide
enough support to stop the penetration. As the
hull becomes out-of-level, the legs will
experience increased transverse load and bending
moment transferred to the hull mostly by the
guide. With the increased guide loads, some
braces will experience large compressive loads.
During normal preload operations it is important
to keep the weight of the hull, deck load, and
preload as close to the geometric center of the
legs as possible, as this will assure equal
loading on all legs. Sometimes, however,
single-leg preloading is desired to increase the
maximum footing reaction of any one leg. This is
achieved by selective filling/emptying of preload
tanks based on their relative position to the leg
being preloaded. Preload is water taken from the
sea and pumped into tanks within the hull. After
the preload is pumped on board, it is held for a
period of time.
41
The Preload Operation is not completed until
no settling of the legs into the soil occurs
during the holding period while achieving the
target footing reaction. The amount of preload
required depends on the required environmental
reaction and the type of Jack Up Unit. Mat Units
normally require little preload. JACKING TO FULL
AIR GAP OPERATIONS Once Preload Operations are
completed, the Unit may be jacked up to its
operational air gap. During these operations it
is important to monitor the level of the hull,
elevating system load and characteristics, and
for trussed-leg Units, Rack Phase Differential
(RPD). All of these must be maintained within
design limits. Once the Unit reaches its
operational air gap, the jacking system is
stopped, the brakes set, and leg locking systems
engaged (if installed). The Unit is now ready to
begin operations. ELEVATED OPERATING
CONDITION When the Unit is performing
operations, no particular differences exist
between the various types of Units. Likewise,
there are no particular cautionary measures to
take other than to operate the Unit and its
equipment within design limits. For Units with
large cantilever reach and high cantilever loads,
extra care must be taken to ensure that the
maximum footing reaction does not exceed a
specified percentage of the reaction achieved
during preload.
42
ELEVATED STORM SURVIVAL CONDITION When the Unit
is performing operations, the weather is to be
monitored. If non-cyclonic storms which exceed
design operating condition environment are
predicted, Operations should be stopped and the
Unit placed in Storm Survival mode. In this mode,
Operations are stopped, equipment and stores
secured, and the weather and watertight
enclosures closed. If cyclonic storms are
predicted, the same precautions are taken and
personnel evacuated from the Unit. This is how a
jack up rig is bought from the shore to the
required location for drilling.
43
ELEVATING SYSTEM All Jack Ups have mechanisms
for lifting and lowering the hull. The most basic
type of elevating system is the pin and hole
system, which allows for hull positioning only at
discrete leg positions. However, the majority of
Jack Ups in use today are equipped with a Rack
and Pinion system for continuous jacking
operations. There are two basic jacking systems
Floating and Fixed. The Floating system uses
relatively soft pads to try to equalize chord
loads, whereas the Fixed system allows for
unequal chord loading while holding. There are
two types of power sources for Fixed Jacking
Systems, electric and hydraulic.Both systems have
the ability to equalize chord loads within each
leg. A hydraulic-powered jacking system achieves
this by maintaining the same pressure to each
elevating unit within a leg. Care must be taken,
however, to ensure that losses due to piping
lengths, bends, etc., are either equalized for
all pinions or such differences are insignificant
in magnitude. For an electric powered jacking
system, the speed/load characteristics of the
electric induction motors cause jacking motor
speed changes resulting from pinion loads, such
that if jacking for a sufficiently long time, the
loads on any one leg tend to equalize for all
chords of that leg.
44
ELEVATING SYSTEM
GUIDES
45
UPPER AND LOWER GUIDES All Jack Ups have
mechanisms to guide the legs through the hull.
For Units with Pinions, the guides protect the
pinions from bottoming out on the rack teeth.
As such, all Units are fitted with a set of upper
and lower guides. Some Jack Up Units, which have
exceptionally deep hulls or tall towers of
pinions, also have intermediate guides. These
guides function only to maintain the rack the
correct distance away from the pinions and are
not involved in transferring leg bending moment
to the hull. Guides usually push against the tip
(vertical flat side) of the teeth, but this is
not the only form of guides. There are also other
forms of guides such as chord guides, etc.
Depending on accessibility, some guides are
designed to be replaced and are sometimes known
as wear plates.In addition to protecting the
pinions and hull, all upper and lower guides are
capable of transferring leg bending moment to the
hull to some degree determined by the design. The
amount of moment transferred by the guides to the
hull as a horizontal couple is dependant on the
relative stiffness of the guides with respect to
the stiffness of the pinions and/or fixation
system (if any).
46
DRILLING RIG COMPONENTS
1. Crown Block and Water Table 2. Catline Boom
and Hoist Line 3. Drilling Line 4. Monkeyboard 5.
Traveling Block 6. Top Drive 7. Mast 8. Drill
Pipe 9. Doghouse 10. Blowout Preventer 11. Water
Tank 12. Electric Cable Tray 13. Engine Generator
Sets 14. Fuel Tank 15. Electrical Control
House 16. Mud Pumps 17. Bulk Mud Component
Tanks 18. Mud Tanks (Pits) 19. Reserve Pit 20.
Mud-Gas Separator 21. Shale Shakers 22. Choke
Manifold 23. Pipe Ramp 24. Pipe Racks 25.
Accumulator
47
Crown Block and Water Table
An assembly of sheaves or pulleys mounted on
beams at the top of the derrick. The drilling
line is run over the sheaves down to the hoisting
drum.
Catline Boom and Hoist Line
A structural framework erected near the top of
the derrick for lifting material.
Drilling Line
A wire rope hoisting line, reeved on sheaves of
the crown block and traveling block (in effect a
block and tackle). Its primary purpose is to
hoist or lower drill pipe or casing from or into
a well. Also, a wire rope used to support
the drilling tools
48
Monkeyboard
The derrickman's working platform. Double board,
tribble board, fourable board a monkey board
located at a height in the derrick or mast equal
to two, three, or four lengths of pipe
respectively.
Traveling Block
An arrangement of pulleys or sheaves through
which drilling cable is reeved, which moves up or
down in the derrick or mast.
Top Drive
The top drive rotates the drill string end bit
without the use of a kelly and rotary table. The
top drive is operated from a control console on
the rig floor.
49
Mast
A portable derrick capable of being erected as a
unit, as Distinguished from a standard derrick,
which cannot be raised to a working position as a
unit.
Drill Pipe
The heavy seamless tubing used to rotate the bit
and circulate the drilling fluid. Joints of pipe
30 feet long are coupled together with tool
joints.
Doghouse
A small enclosure on the rig floor used as an
office for the driller or as a storehouse for
small objects. Also, any small building used as
an office or for storage.
50
Blowout Preventer
A large valve, usually installed above the ram
preventers, that forms a seal in the annular
space between the pipe and well bore or, if no
pipe is present, on the well bore itself.
Water Tank
Is used to store water that is used for mud
mixing, cementing, and rig cleaning.
Electric Cable Tray
Supports the heavy electrical cables that feed
the power from the control panel to the rig
Motors.
51
Engine Generator Sets
A diesel, Liquefied Petroleum Gas (LPG), natural
gas, or gasoline engine, along with a mechanical
transmission and generator for producing power
for the drilling rig. Newer rigs use electric
generators to power electric motors on the other
parts of the rig.
Fuel Tanks
Fuel storage tanks for the power generating
system.
Electric Control House
On diesel electric rigs, powerful diesel engines
drive large electric generators. The generators
produce electricity that flows through cables to
electric switches and control equipment enclosed
in a control cabinet or panel. Electricity is fed
to electric motors via the panel.
52
Mud Pump
A large reciprocating pump used to circulate the
mud (drilling fluid) on a drilling rig.
Bulk Mud Components in Storage
Hopper type tanks for storage of drilling fluid
components.
Mud Pits
A series of open tanks, usually made of steel
plates, through which the drilling mud is cycled
to allow sand and sediments to settle out.
Additives are mixed with the mud in the pit, and
the fluid is temporarily stored there before
being pumped back into the well.
53
Reserve Pits
A mud pit in which a supply of drilling fluid has
been stored. Also, a waste pit, usually an
excavated, earthen - walled pit. It may be lined
with plastic to prevent soil contamination.
Mud-Gas Separator
A device that removes gas from the mud coming out
of a well when a kick is being circulated out.
Shale Shaker
A series of trays with sieves or screens that
vibrate to remove cuttings from circulating fluid
in rotary drilling operations. The size of the
openings in the sieve is selected to match the
size of the solids in the drilling fluid and the
anticipated size of cuttings. Also called a
shaker.
54
Choke Manifold
The arrangement of piping and special valves,
called chokes, through which drilling mud is
circulated when the blowout preventers are closed
to control the pressures encountered during a
kick.
Pipe Ramp
An angled ramp for dragging drill pipe up to the
drilling platform or bringing pipe down off the
drill platform.
Accumulator
The storage device for nitrogen pressurized
hydraulic fluid, which is used in operating the
blowout preventers.
55
BASIC SYSTEMS IN JACK UP RIGS
  • Drilling Systems
  • Drilling system is the heart of the jack up rig.
    Drilling is carried out at the drill floor which
    is at certain elevation from the main deck. There
    is a mouse hole and rat hole in which the drill
    stems are assembled. Once the stem is assembled
    it is placed concentric with the rotary table
    which is either driven from the top or from the
    bottom. Kelly bushing is provided to support the
    drill stem and prevent it from buckling.
  • Mud Systems
  • These are of two types high pressure mud system
    and low pressure mud system.
  • Mud is a mixture of raw water, clay, bainite and
    some other minerals. Working of the jack up rig
    depends upon the power of the mud system. It is a
    cyclic procedure which is used to convey the
    crushes from the bottom of drill bits to the top
    of mud tanks. This is called high pressure mud
    system. when we punch through the reservoir a
    high pressure builds up due to the presence of
    gases, to prevent our system from blowing out
    high viscosity mud is used to lower the pressure.
    When the mud returns from the BOP and goes to
    shale shaker assembly, it is called low pressure
    mud system

56
MUD SYSTEM
CATERPILLAR GENERATOR
57
  • Power Generation Systems
  • Power is must to run a jack up rig. Jack up rigs
    are installed in deep seas thereby we have no
    other provision of getting power. Hence the
    internal combustion engines are used to supply
    power. They are high capacity engines which are
    located in engine rooms at the main deck. Once
    the electricity suddenly shuts off then the crude
    oil in the annular should be at the same level of
    the mud which is present in the drill stem below
    the return line.
  • Cementing Systems
  • Cementing system is a provision which is used to
    cement the sides of the well forbidding the soft
    soil to enter the drill well.
  • In cement system there are four tanks which have
    an accumulator on the top which mixes the cement
    continuously with raw water. On both sides of the
    rig there are centrifugal pumps which sucks
    marine water from the sea and supply it to the
    tank which is used to dilute the cement.
  • Living quarters Landing Systems
  • Living quarters are provided for the officials
    and a helideck is also commissioned for some
    external support needed for the persons working
    on the rig. Complete care of the people is taken
    to ensure a safe working environment.

58
BOP
Diesel Electric Generator
59
  • BOP Well Completions
  • BOP stands for blow out preventer. It is the most
    important part of the jack up rig. When the crude
    oil comes from the well it at such pressure which
    can send a rocket to space i.e about 10,000 psi
    so it can destroy the jack up rig in one blow
    ,hence to prevent the rigs we have BOPs which
    cuts the line when a limiting pressure value is
    reached , hence saving the rig.
  • Well completion process, when the drilling and
    cementing is done a plug is placed at the cement
    sleeve which at some clearance from the sea bed.
    The Christmas tree is placed on this plug and the
    plunger on the bottom of Christmas tree is used
    for the production of oil.
  • Jacking and Hydraulic Systems
  • Care must be taken when positioning a new jack up
    rig at a site previously occupied by another jack
    up because of the tendency of the spud cans of
    the new rig to slip into the spud cans holes or
    foot prints left on the sea floor by the
    previous rig. If there is an overlap of a spud
    can over an old spud can hole, there is a
    tendency for the spud can not to penetrate
    straight into the soil, but instead to slip into
    the old spud can hole. This movement of a spud
    can, without a corresponding movement of all the
    other spud cans in the same direction, will
    impose a bending moment on the legs. This bending
    moment can be quite severe and may damage the leg
    in the preloading or jacking up process or it may
    reduce the allowable
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